faq 9

Why do I get negative values when photon counting? I am using a photon counting package (P25P-06) with a CT2. I am detecting at 200nm wavelength and getting negative values. What is the problem ?

There are a few reasons for your system not working:

1. If you have an air path between light source and photomultiplier window then maybe the light is being absorbed before it reaches the photomultiplier.
2. The strength of the light might be so high that the electronics is saturated.
3. The photomultiplier has lost its vacuum. The window should have a yellow colour. If it is clear then this indicates loss of vacuum.

The customer advised that counts were measured without the light source and confirmed that it was the extremely high light levels that totally overloaded the package electronics. This resulted in spurious results.

This scenario highlights how sensitive photomultipliers are at detecting light. This is compounded by the fact that the human eye cannot even make a judgement as to the brightness of the light.

Regards,

Photomultiplier Doctor.

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